Ethylene is a ubiquitous building block in the manufacture of a wide variety of chemical and plastic products. Ethylene is typically produced industrially by pyrolysis of hydrocarbons in a furnace in the presence of steam. The furnace effluent stream comprising a range of components is typically cleaned up, dried to remove water, compressed and passed to an olefins recovery section to condense the ethylene and other condensable heavy end components (ethane, propylene, propane, etc.). The condensed stream is then distilled to remove the light ends (methane and hydrogen) and fractionated to separate ethylene from the heavy ends.
Compositional range of the furnace effluent stream depends on several factors including the type of hydrocarbon feedstock used. A representative composition of the effluent of a furnace employing three different hydrocarbon feedstocks and operated to maximize ethylene formation is given in Table 1.
TABLE 1 ______________________________________ Effluent Composition (mole %) Furnace Feedstock Component Ethane Propane Naphtha ______________________________________ H.sub.2 35.9 20.5 15.8 CH.sub.4 6.5 27.8 26.5 C.sub.2 H.sub.4 34.3 32.0 33.6 C.sub.2 H.sub.6 + 23.3 19.7 24.1 ______________________________________
The condensation of ethylene from the hydrogen and methane components requires considerable refrigeration which makes up a significant portion of the process energy requirements. Where hydrogen, methane and ethylene are recovered by condensation, refrigerants typically include propylene as well as ethylene and sometimes methane. Ethylene and methane refrigeration are generally colder than propylene refrigeration, and have higher energy requirements and require more expensive materials of construction such as, for example, nickel alloys or stainless steel.
Recently, solvent-based olefins recovery has attracted attention as a potential alternative to condensation recovery. For example, Lam et al., "Advanced Ethylene Process", A.I.Ch.E. Spring National Meeting, Session No. 18, Mar. 31, 1993, claims that solvent-based olefins recovery can reduce energy and capital requirements, including such benefits as elimination of the ethylene refrigeration machine, ethylene refrigeration chillers and stainless steel/alloy piping associated with this equipment.
Olefin recovery using absorption in a solvent is described in Lam et al. mentioned above, as well as U.S. Pat. No. 5,220,097 to Lam et al. and U.S. Pat. Nos. 5,019,143 to Mehrta and 4,743,282 and 4,832,718 to Mehra, all of which are hereby incorporated herein by reference. Briefly, methane and hydrogen are separated from an olefin stream by contacting the olefin stream in an absorber with a solvent capable of absorbing the olefins. The olefins then are recovered from the solvent by thermal regeneration, typically in a reboiled regeneration column to vaporize the olefins which are condensed overhead for further processing and purification. The hydrogen and methane are recovered as an overhead vapor from the absorption unit, and can be further processed to obtain purified hydrogen and methane streams by cryogenic fractionation and/or methane extraction with a solvent.
A recently disclosed embodiment of the absorption method commercially offered is described in Lam et al. Briefly, a front-end heat-pumped deethanizer or depropanizer and a selective acetylene hydrogenation system are combined with a solvent absorption system to recover the ethylene product. Using a full range naphtha feedstock with a front-end depropanizer system, the pyrolysis furnace effluent is indirectly quenched in transfer-line exchangers and then directly quenched in an oil quench tower and a water quench tower with conventional heat recovery. The cooled water quench tower overhead stream typically is compressed in three stages to an optimum pressure primarily governed by the operating pressure of the front-end depropanizer. At the cracked gas compressor third stage discharge, acid gases are removed by caustic scrubbing. The acid gas-free cracked gas is then dried before entering the fractionation section of the plant. A low pressure debutanizing stripper may be located in the compression train to remove pentane and heavier components from the cracked gas.
In the Lam et al. process, the front-end heat-pumped depropanizer allows fractionation at low pressure and condensation at high pressure. Fouling is said to be minimized when the depropanizer is operated at low pressure. The energy for heat pumping of the depropanizer is provided by the fourth stage of the cracked gas compressor. At the compressor discharge, acetylene is selectively hydrogenated to ethylene in the front-end reactor system. In addition, about 80% of the methyl acetylene and about 20% of the propadiene are said to be selectively converted to propylene.
The acetylene-free propane and lighter portion of the cracked gas in the Lam et al. solvent absorption system leaves the reactor and is dried in the secondary drier to remove trace quantities of moisture, leaves the depropanizer reflux drum and is fed to a reboiled absorber column. The ethylene and heavier components are absorbed by the solvent while methane and lighter components, together with some ethylene, leave the top of the absorber. This overhead stream is then fed to a small demethanizer section where essentially all of the ethylene and heavier components are recovered. The demethanizer section is autorefrigerated by means of an expander, and no external refrigeration is said to be required. The rich solvent is fed to a solvent regenerator where the demethanized C.sub.2 's and C.sub.3 's are recovered as overhead product. The lean solvent is returned to the absorber after heat recovery.
The C.sub.2 's and C.sub.3 's are further separated in a conventional deethanizer to produce C.sub.2 and C.sub.3 fractions. These two fractions are then processed in their respective superfractionators to produce polymer grade ethylene and propylene products. Ethane and propane bottom products are said to be recycled and cracked to extinction in the pyrolysis furnace. Refrigeration for the entire ethylene recovery process is said to be supplied by a propylene refrigeration compressor only and no ethylene or methane refrigeration is said to be required. The hydrogen recovery section can also include a demethanizer for separating components heavier than methane from the liquid methane stream and forming a methane vapor stream for expansion in the expansion zone. Compared to state-of-the-art conventional demethanizer-first condensation-based olefins recovery process, the above-described Mehra process still uses more compression power and has a higher low pressure steam requirement. Also, the Mehra process uses a high solvent circulation rate. In order to achieve about 99.8% recovery of ethylene without the use of ethylene refrigeration, 75-80% of the hydrogen product must be expanded to fuel gas pressure to provide refrigeration in the demethanizer area.
Accordingly, there is a need for an absorption-based olefins recovery process which has reduced energy requirements, eliminates the need for ethylene refrigeration, and reduces the solvent circulation rate for absorption.